Transcript PowerPoint Presentation - Lecture 4: Ecology of Evolution cont`d

Plant Adaptations
Outline:
•Photosynthesis and respiration
•Environmental controls on photosynthesis
•Plant adaptations to:
–High and low light
–Water limitation
–Nutrient availability
Readings: Chapter 6
Conditions and Resources
• Conditions are physical / chemical
features of the environment
– E.g. Temperature, humidity, pH, etc.
 Not consumed by living organisms (but may
still be important to them)
• Resources are consumed
– Once used, they are unavailable to other
organisms
– Plants: sunlight, water, mineral nutrients, …
– Animals: prey organisms, nesting sites, …
Plant Resources
• Plants are autotrophs - make their own
organic carbon form inorganic nutrients
– Need light, ions, inorganic molecules
• Plants are sessile
– Grow towards nutrients
PHOTOSYNTHESIS
Conversion of carbon dioxide into simple sugars
LIGHT
6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O
Light reactions
Dark reactions
carboxylation
Photosynthetically Active Radiation, PAR
RESPIRATION
C6H12O6 + 6O2  6CO2 + 6H2O + ATP
Net photosynthesis = Photosynthesis - Respiration
Photosynthesis
involves gas
exchange
Controls on photosynthesis
•Light
•Water
•Nutrients
•Temperature
1. Light
PAR
Tradeoff
• Shade plants grow
better in the sun
than in the shade,
• but sun plants grow
faster than shade
plants in direct sun
Shade
plant
Sun
plant
Tradeoff
• Shade plants
survive well in either
sun or shade
• Sun plants cannot
tolerate shade
Shade
plant
Sun
plant
• 9 tree species of
Macaranga from
Borneo, Malaysia
Phenotypic plasticity
• Most plants have the ability to alter their
morphology (within limits) in response to
light conditions
Phenotypic plasticity
• Sun and shade leaves can exist within the
same tree
More deeply lobed
--> More rapid heat
loss
Sun leaf
• thicker
• more cell
layers
• more
chloroplasts
Shade leaf
• flat
• thin
• larger
surface area
/ unit weight
Sun leaves
Shade leaves
•Leaves at many angles
•High saturation point
•High compensation point
•Produce more RUBISCO
•Horizontal leaves, single layer
•Low saturation point
•Low compensation point
•Produce less RUBISCO
•High respiration
•Less chlorophyll
•RUBISCO availability limits
photosynthesis rate
•Low respiration
•More chlorophyll
•Light availability limits
photosynthesis rate
2. Water
Transpiration
For transpiration to occur
atmosphere < leaf < root < soil
Water potential
w = p +  + m
p= = hydrostatic pressure
 = = osmotic pressure
m= = matric pressure
Stomata
• Reduction in soil  --> stomata close
• Species differ in tolerance to drying soils
Strategies for drought
i.
Avoiders
•
•
•
•
Short lifespan
Wet season
Seeds survive drought
Drought deciduous species
–
Leaves shed in dry season
Strategies for drought
ii. Tolerators
•
•
•
Leaves transpire slowly
Change orientation of leaves
Sunken stomata
–
•
E.g. pines
More efficient photosynthesis
•
•
E.g. C4 --> reduces photorespiration
E.g. CAM --> stomata open at night
C4
photosynthesis
CAM
photosynthesis
C4
CAM
CAM
% of grasses that are C4
Water absorption
• Root hairs increase surface area
• Structure of the root
system varies
between species,
depending on the
amt. of soil moisture
in their env’t
• Individual species
show phenotypic
plasticity
• wet soil -->
shallow roots
near surface
(greater oxygen
availability)
• dry soil --> deep
roots
3. Nutrients
• Macronutrients – needed in large
amounts (e.g. C, H, O, … N, P, K, Ca, Mg,
S)
• Micronutrients – trace elements (e.g.
Fe, Mn, B)
• Micro/macro refer to the quantity
needed
Table 6-1
Nutrient uptake rates
• Reach plateau with increasing nutrient
concentration
Maximum growth rate of a plant reflects N availability in its
natural habitat. A. stolonifera occurs on more nitrogen-rich
soils than A. canina.
Evergreen leaves
• Plants adapted to nutrient-poor conditions
tend to have evergreen leaves
4. Effects of temperature
= Condition
• Increase temperature --> increase
biochemical reaction rate
• At high temperature,
enzymes denature
--> death
• Gross photosynthetic rate
increases up to a point with
increasing temperature
• Respiration rate also
increases with temperature.
• Net photosynthesis is
maximal at a point slightly
below that at which gross
photosynthesis is maximal
Leaf temperature
•
•
> 95% of sunlight absorbed by a leaf
becomes heat
Cooling of leaves:
1. Transpiration
2. Convection (movement of cool air around a
leaf)
C4 plants
• Have higher temperature optima than C3
Phenotypic plasticity
• Individual species can modify their Topt
according to the changing seasons
= acclimatization
Response to cold
Chilling injury
Freezing
- near, > 0 oC
- cell membranes rupture
- < 0 oC
- ice inside cells = death
- ice outside cells = dehydration
(may survive)
-may kill juveniles only
Saguaro cacti (S.W. United States) store large amounts of water; they can
tolerate short periods of freezing temperatures
CLOSER TO HOME
•Freeze-tolerant plants: frost hardening
•When T decreases – plants synthesize sugars, amino
acids, other molecules to act as antifreeze.
•Winter – deciduous plants
•Lose leaves in autumn
•Leaves very efficient in summer – high
photosynthesis rate
•Leaves can’t survive freezing
•Costly in energy, nutrients to rebuild leaves
•Chilling breaks seed dormancy for
temperate/boreal spp.
•Germinates only in spring
Plants are phenotypically plastic